1. Field of the Invention
The present invention relates to a superconducting thin film of compound oxide material deposited on a substrate, more particularly to a novel layered structure of a superconducting thin film of compound oxide material which exhibits substantially no anisotropy in the critical current density (Jc) against an external magnetic field. The present invention provides also a process for preparing such thin film.
In the specification, a superconducting thin film of compound oxide material means a thin film made of superconducting compound oxide possessing the critical temperature above 30K.
2. Description of the Related Art
A superconducting compound oxide material of (La, Sr).sub.2 CuO.sub.4 which exhibit the superconductivity at 30K was discovered in 1986 by Bednorz and Mailer (Z. Phys. B64, 1986 p 189). Another superconducting material of YBa.sub.2 Cu.sub.3 O.sub.7 -d having the critical temperature of about 90K was discovered, in the United States of America, in 1987 by C. W. Chu et al. (Physical Review letters, Vol. 58, No. 9, p 908). Maeda et al discovered so-called bismuth type superconducting material of Bi--Sr--Ca--Cu--O (Japanese Journal of Applied Physics. Vol. 27, No. 2, p 1209 to 1210), After then, a variety of new high-temperature compound oxide systems are discovered.
The superconducting compound oxide materials such as Y--Ba--Cu--O system, Bi--Sr--Ca--Cu--O system, Tl--Ba--Ca--Cu--O system or the like are expected to be utilized in electronics devices such as Josephson element or superconducting transistors due to their high critical temperatures (Tc). In order to realize such electronics devices, it is indispensable to prepare thin films of these oxide superconductors.
Recently, it has become possible to prepare thin films of single crystal of good quality from these superconducting compound oxide materials on a single crystal substrate of MgO, SrTiO.sub.3 or the like by physical vapour deposition (PVD) technique including sputtering method, vacuum evaporation method and laser abrasion method or by chemical vapour deposition (CVD) technique.
It is known that crystals of these high-temperature compound oxide superconductors show anisotropy in their superconducting properties. In fact, their crystals show a higher value of critical current density (Jc) along the direction which is perpendicular to c-axis than the other two directions of a-axis and b-axis. Therefore, in the case of a c-axis oriented thin film of superconducting compound oxide material, electric current flows mainly in parallel with a surface of the thin film, while, in the case of an a-axis oriented or b-axis oriented thin film thereof, electric current flows mainly along a depth direction of the thin film. Still more, the superconducting compound oxide materials exhibit another anisotropy in their coherent length. Namely, the coherent length along a-axis is greater than that of along c-axis [the coherent length along c-axis is several angstroms (.ANG.) while the coherent length along a-axis is several ten angstroms (10 .ANG.)]. Therefore, either the a-axis (or b-axis) oriented thin film or the c-axis oriented thin film is selected according to applications.
The thin films of superconducting compound oxides exhibit also "magnetic field-induced anisotropy" in their critical current density (Jc). In other words, the critical current density (Jc) of a superconducting thin film of compound oxide material, in particular c-axis oriented thin film thereof, is influenced severely by an external magnetic field applied. In fact, when the critical current density (Jc) of a c-axis oriented superconducting thin film of compound oxide material is measured in an external magnetic field, it is confirmed that the critical current density Jc(V) which is measured in a magnetic field applied perpendicularly to a surface of the thin film is smaller than the critical current density Jc(P) which is measured in a magnetic field applied in parallel with the surface of the thin film. No theoretical explanation of the "magnetic field-induced anisotropy" can be given at this stage. In any way, such "magnetic field-induced anisotropy" is not desirable in certain applications in which stability or resistance to the external magnetic field is mostly required.